Given that I'm probably as much of an expert as anyone (translation: I'm picking this out of me arse), the first thing to come to mind with the concept of "Enterprise Software" is software that is needed for business-critical processes. Of course, this definition is probably over-broad. After all, it would include the word processor that the secretary uses (after all, he couldn't get his job done without it - it's critical to his part of the process), the email program that the manager uses (again, she couldn't do her job without it since about all she does is send and receive email... at least at my company that's all managers do...), or even the web-browser that everyone uses - since in order to access the web-based enterprise software, one needs a browser. Now, granted, with commoditised browsers, if one browser stops working, you do, in theory, have a few other options available to you. In actual practice, however, I'm not sure - how many apps depend on IE, and depend on the quirks of IE?
For example, we recently switched over to Rational ClearCase/ClearQuest for development and tracking. ClearCase itself works on many platforms. ClearQuest doesn't. And I don't work on Windows. So I started using the CQWeb interface - which only supported IE on Windows and MacOS. It has been an uphill battle to get Firefox on Linux supported - although they seem to be going that way. For me to get through my processes, I need CQWeb - to me, that makes it Enterprise Software.